1. Field of the Invention
The invention concerns a dynamic scale with multiple weighing pans and a method to operate such a dynamic scale. The invention allows an increased throughput of products of different dimensions that are to be weighed, in particular of unit loads to be weighed (for example of mail pieces and other shipments) but also of weighed goods which have no permanent, fixed dimensions. The invention is suitable for use in weighing systems, in franking systems with dynamic scales, or in mail systems of different delivery services, mail carriers and shippers which have a dynamic scale.
2. Description of the Prior Art
From EP 20 65 686 A1 (=US 2009/0139777 A1), a parallel arrangement of two weighing units is known in which the throughput can be increased via parallelization of the weighing process. However, additional devices to separate and merge the mail flow before and after the scale are necessary for this, which increases the space requirement. The parallel arrangement of two weighing units brings with it the problem of the markedly increased space requirement due to the two units to control the mail flow, a markedly increased control expenditure and higher costs.
From US 2009/032311 A1 (=WO 2007/057001 A1) (“Method to weigh with multiple weighing cells”), a device is known that avoids production lines that are specifically designed for the dimensions of objects to be weighed. The weighing of objects of different dimensions is enabled by the weighing units being arranged essentially transverse to the transport direction, with the products (goods, weighed goods) resting flat on the transport device. Because the products are weighed with their largest dimension aligned transverse to the transport direction, the distance between the goods is reduced, which leads to an increased throughput of the weighed goods given the same transport speed. The weighing units arranged in the same plane transverse to the transport direction are all of identical design and are oriented with their base plate on the same table plate. Each weighing unit has a weighing cells and a weighing pan with a transport device that is arranged on the weighing pan of the dynamic scale. In this method, the weighing signals of the weighing units must be coupled with one another and complicated calculation methods are used since a portion of the weight of the weighed good is distributed among the respective weighing units.
From EP 2017589A2 (=US 2009/008859 A1), a system is known in which at least two weighing units are connected in series. Short letters can be weighted at short intervals by means of one of the weighing units; given long letters, the weighing result is a superposition of the weighing results of both weighing units. The higher cost relative to a system with only a single weighing unit is disadvantageous. The arrangement of multiple weighing units in series makes a markedly increased computing cost necessary in the calculation of the weight values, which can be time-critical. Slippage/synchronization programs can occur between the weighing units, which can lead to impacts that affect the weighing result.
In the field of dynamic weighing in mail processing, various solutions from different vendors are already known. The companies Neopost and Pitney Bowes use weighing systems for flat weighing goods (mail pieces) supplied lying flat on their back sides so that a franking imprint can be applied on the front side of the mail pieces.
A device to weigh moving mail goods, in particular mail pieces (letters) arranged standing on an edge, is known from European Patent EP 974 81 B1. A transport belt driven by a motor runs on track rollers across a weighing plate that is equipped with a guide wall to guide the mail pieces. The guide wall is inclined at an angle of 108° relative to the table plate.
The mail pieces tilted on their front sides slide further along the guide wall while the approximately horizontally arranged transport belt on which the mail pieces standing on edge are transported is moved further. Mail pieces means, for example, post cards, enveloped letters and other mail goods up to 20 mm thick. The throughput in the latter cited system amounts to approximately 50 letters per minute given dynamic weighing of the mail goods. The throughput of a franking machine is determined from the number of letters per time unit and amounts at most to 150 letters per minute. At a given transport speed and a letter interval from the leading letter edge of the one enveloped letter to the leading letter edge of the next enveloped letter (edge-to-edge), the throughput results from the quotient of the transport speed and the letter interval. The motor, the transport belt with track rollers and the weighing plate with guide wall load a weighing cell. At the weighing cell, the weighing plate with the transport belt and the track rollers as well as a first motor are arranged so that the weighing cell is loaded with an initial load. A first sensor is arranged at the intake of the weighing unit, which first sensor outputs a first signal to a control unit which starts the weighing process for a mail piece (letter). Moreover, a second sensor is provided at the outlet of the weighing unit, which second sensor outputs a second signal to a control unit which generates a message that a subsequent letter following the letter can be supplied. The aforementioned components form the dynamic scale that, together with a franking machine of the Jetmail® type, is operated in a franking system of the applicant with a system speed which concurrently determines the throughput of mail pieces.
In an advanced extended version, a franking system from Francotyp-Postalia GmbH has at least one automatic feed station, a dynamic scale and a franking machine located downstream in the mail flow. In the Centormail®-type franking machine, a transport system is used in which successive mail pieces standing on an edge are transported. Given mail pieces such as letters, a letter spectrum with letter lengths from 140 mm to 353 mm can be processed. Letters in the C6 length format thus have an average letter length of 229 mm. Customer requirements for a higher throughput of the franking system with the Centormail® franking machine make it necessary that either the transport speed for mail pieces (in particular for letters) in the system is increased, or the distance between the letters is reduced.
A method and arrangement to determine a weight with a dynamic scale are known from the European Patent 1014052 B1. The arrangement has a transport device, a weighing plate, a weighing cell, sensors and an electronic control unit which—in addition to a control of the transport device—implements an evaluation of the measurement values communicated by the weighing cell and the output of a corrected weight value to a franking machine via an interface. The throughput of the aforementioned system is limited to approximately 50 letters per minute. Although the throughput could be even further increased with a higher transport speed (at the cost of the precision of the weighing value and reliability of the mail piece transport), the precision of the weighing value and reliability of the mail piece transport should specifically not be reduced. When the dynamic scale is used as a peripheral device for a franking system, the franking system limits the transport speed of a mail piece. An increase of the system speed is not possible, or is possible within very narrow, very precisely considered limits. Limiting factors are possibly also provided by other peripheral devices of the franking system, for example a separation certainty of an automatic mail piece separator and feed station that decreases with increasing speed. For example, components (motors, gears, loads) and service life matched to and protected by the present system speed of the Centormail® franking system are used. Additional limitations also result from the processing times, for example the calculation speed, and the billing time for franking imprints in the dynamic weighing mode of the Centormail® franking system. The increasing customer requirements for the throughput of the system make it necessary to find new designs.